I've noticed some fatal error messages when I attempt to use the Previous / Next Thread buttons, and when I try to view posts by people who are no longer members. It doesn't happen all the time, but it is frequent. A specific instance:

It appears that if I'm in any reply to an initial post, the Next Thread button returns this error:

A fatal error has occurred:

Undefined subroutine &GForum::User::blank_user called at /home/perlguru/webdocs/gfadmin/GForum/Post/View.pm line 324.

Please enable debugging in setup for more details.

Everything works fine in flat mode. Perhaps that'll help solve the problem, or at least point you in the right direction. "...there's some good in this world, Mr. Frodo, and it's worth fighting for." - Samwise Gamgee, The Two Towers